Samsung reverses the position of Android's Back and Menu (aka Options) buttons, when compared to almost every other device.
How does one swap the functionality of these hardware buttons, specifically on the Samsung Galaxy S4?
Fortunately, the hardware buttons have no visual indication of the pre-programmed functionality if the user turns off the LED lights for those buttons. Thus, swapping them so the Back button is on the left should be transparent to the user.
Root and Xposed solutions are fine, if required.